Someone correct me if I'm wrong, but I think it's totally unethical and unconventional to ask someone which school they prefer. I don't think it's done.
When Penn called me with my acceptance my interviewer did ask me if I thought I'd be attending, but I think that was for 2 reasons:
1. I do think they're told to get a feel for whether or not we'll be coming, in order to try to guestimate class size as well as determine if they want to offer some financial incentive to the applicants they really want to retain (not me haha, but I know a couple of people who played it cool and had some $$ thrown at them)
2. I think the fact that I was noticeably jumping up and down screaming and on the verge of tears was too much excitement for her to ignore and she wanted to get in on the party . She sounded almost as excited as I did when she asked.
I didn't interview anywhere else so I can't speak for other schools...but I think its generally not acceptable to ask an applicant where they want to go the most (asking why they like your school, on the other hand, is fair game... so know what you do like about that school)